The theoretical principles underlying the operation of waveguides and related devices is well known to those of skill in the art. Design and fabrication of practical devices on the other hand, has been difficult and is the object of ongoing research. Due to material properties and the demanding parameters involved, fabrication of one of the preferred type of waveguiding devices, single mode waveguides, is particularly challenging and may often be the result of trial and error procedures to obtain the desired end product. Refractive index differences of less than one percent variance from design values are often required as will be appreciated from the discussion that follows.
A dispersion constant b, defined in equation one (1) below was calculated for a slab waveguide (i.e. two-dimensional waveguide) consisting of three (3) planar layers; an upper and lower cladding layer and a core therebetween. For purposes of calculation, the core thickness was taken as four (4) microns and the upper and lower cladding layers had a refractive index of 1.630. The core index was varied from 1.630 to 1.710 to generate a series of dispersion curves as a function of index difference between core and cladding. For purposes of convenience a propagation constant b is utilized where: ##EQU1## and N eff is the effective index of the mode, Ncl and Nco are the cladding and core indices respectively. The results of the calculation are shown in FIG. 1.
As may be seen, a structure with a four (4) micron thick core will quickly convert from a single mode device to a dual mode device when the index difference exceeds about 0.007. This would have a devastating effect on bandwidth and operating characteristics of practical single mode devices. In general, the problem is aggravated in systems with polymeric devices which are poled to non-centrosymmetric structures during fabrication since the poling step may introduce index changes in the various layers of the device. For example, polymeric electro-optic materials are known to change refractive index, or more accurately, refractive properties, upon poling to a stable non-centrosymmetric structure. As used hereinafter refractive index refers to the index presented to the transverse electric mode of the light travelling in a waveguide, that is, the field in the general direction of arrows 30 in FIG. 2.